One friend was granted immunity from prosecution by the Solicitor-General, Dr David Collins, QC, in return for giving evidence.
Mr Benefield said there was insufficient evidence to charge anyone else with an offence, such as accessory to murder after the fact or perverting the course of justice, in connection with the death.
"We were disappointed that some people who knew what Brad Callaghan had done did not come to police earlier in the investigation," said Mr Benefield.
"Had they done so, they could have saved Carmen's family from a small amount of heartache."
He also criticised the "extreme media intrusion" which at times complicated the investigation and diverted police attention.
Callaghan was a "person of interest" to police as soon as he reported Ms Thomas missing on July 12, said Mr Benefield, as was often the case until evidence proved otherwise.
"There are numerous cases where the person who has reported a person missing is subsequently identified as an offender connected with or responsible for the missing person's demise."
But Callaghan was always "high on the list" of suspects and Mr Benefield said the case developed to the point where no one else could have been responsible for the disappearance and death of Ms Thomas.
While building the case against him, the Operation Keppel inquiry team also had to unravel the sustained efforts to cover his tracks.
He reported her missing nearly two weeks after striking her on the head multiple times, destroying evidence, sending text messages from her phone, disposing of the body in the bush and dumping her car in Hamilton.
The day after she was reported missing, police found Ms Thomas' car in Hamilton and discovered smears of blood inside. A forensic examination of her Ngapuhi Rd flat revealed blood splatter patterns which suggested a vicious assault and matched the blood in the car.
By interviewing Ms Thomas' neighbours, detectives learned that her landlord heard a panicked scream and whimpering coming from inside the flat. The landlord knocked on the door but heard nothing more. Soon after, Callaghan emerged on the driveway.
"[Callaghan] was wearing his boxers, a shirt and was carrying his jeans and shoes in his hands," said the landlord's statement to police.
"He told me that Carmen had vomited up blood and that she had vomited it all over his trousers."
After convincing the landlord that Ms Thomas was sick on the day he killed her, Callaghan texted a friend to say: "Can you please bring 3 black rubbish bags and a bottle me (sic) kerosene.. Keep it quiet".
The pair met at a supermarket in Greenlane, before Callaghan returned to clean Ms Thomas' flat. He wiped away blood splatters and hid evidence - including her body - in a large Auckland Council recycling bin, which he took away in his car. At some point Callaghan swapped it with his neighbour's bin.
In the days after the murder, Callaghan tried to dispose of Ms Thomas' body and make it seem as if she had travelled to Hamilton and not returned home.
He bought a steel meat cleaver and a 120-litre plastic bin.
He also sent text messages from her phone to give the impression she was still alive, even having a text conversation with himself.
Somewhere Callaghan dismembered the body into eight parts and encased them in concrete inside the plastic bin and smaller paint buckets.
After a failed attempt to dump the body at sea with the help of a second friend, Callaghan buried Ms Thomas in a shallow grave in the Waitakere Ranges then drove her car to Hamilton and abandoned it.
The dumping of the car was consistent with the text messages Callaghan sent from her phone that she was travelling south. He also left a note and cash in the car to distract the police inquiry.
The friend who helped buy the cleaning products then drove Callaghan home from Hamilton.
Police tracked Callaghan's movements through cellphone records and CCTV film. Detectives took statements from more than 130 witnesses and gathered enough evidence to arrest Callaghan on September 21, 2010 - 12 weeks after Ms Thomas' death and 10 weeks after he reported her missing.
Her body was found in the Waitakere Ranges on October 1.
Brad's helpers
1. Within minutes of killing Ms Thomas, Callaghan rings this man and arranges to meet him at the Countdown supermarket in Greenlane. Also texted: "Can you please bring 3 black rubbish bags and a bottle me kerosene [sic]..Keep it quiet." The friend purchases the plastic bags and cleaning products.
A week later, Callaghan drives Ms Thomas' car to Hamilton in the early hours and leaves it there. The same friend picks him up. His house was later searched by police but charges were never laid, as there was no evidence that he knew Callaghan had killed Ms Thomas.
2. Four days after killing Ms Thomas, Callaghan arranges to meet this friend at the Okahu Bay boat ramp to go out on the water. He asked whether Callaghan was okay, then pushed him for a response when he was quiet. "Then he said 'I've killed Carmen'."Callaghan then showed him the dismembered body of Ms Thomas, encased in concrete inside plastic containers in the boot of his Subaru stationwagon.
Callaghan told his friend that he needed help to dispose of the body at sea. The two loaded the plastic containers into the boat and went on to the water, but the friend changed his mind and refused to help. But he did not call the police. The man, referred to as Witness 70, was later given immunity against prosecution in return for giving evidence.
3. The day after the failed attempt to dump the body at sea, Callaghan arrived at the man's house shaking and looking stressed. The friend asked if he had "knocked someone up". Callaghan replied: "'Nah, nah mate not knocked up, knocked off' ... That's when he told me he killed Carmen." Callaghan then asked the friend if he would say he'd seen Ms Thomas on July 1, to try to thwart the police. He asked him where the body was and Callaghan indicated it was in the back of his car. "Doesn't it smell? ... He said it was in buckets of concrete and I had the impression it was not in one piece."
Callaghan told him a disposal mission in the Hunua Ranges failed because the spade broke.
He then said he would bury her in the Waitakere Ranges. Callaghan said he thought he would get caught, but wanted to prolong the investigation to gain some time.
